This invention relates to the treatment of chlorinated hydrocarbon process streams to remove and recycle metallic impurities such as ferric iron.
Chlorinated hydrocarbons possess various utilities as, for example, solvents and pesticides and as intermediates in organic synthesis.
A number of valuable chlorinated hydrocarbons such as 1,1-dichloroethane, 1,2-dichloroethane, 1,1,2,2-tetrachloroethane, and 1,1,1-trichloroethane, ethyl chloride and analogous chlorinated derivatives of higher hydrocarbons such as propane or butane are commonly made by liquid phase catalytic hydrochlorination or chlorination of the corresponding unsaturated precursor such as ethylene, vinyl chloride or vinylidene chloride. Metallic halides, particularly ferric chloride, are often the catalysts in these processes. However, the removal of ferric iron or other metallic contaminants that result from the use of these catalysts has been a long-standing problem. The presence of metalic ions, particularly in the form of ferric chloride catalysts, during the flashing and recovery of the desired chlorinated hydrocarbons causes dehydrochlorination of the desired chlorinated hydrocarbons and subsequent polymerization of the resulting unsaturated products. This results in the production of tars which must be disposed of as hazardous wastes.
Various techniques have been proposed to remove the ferric iron or other metallic contaminants from chlorinated hydrocarbon process streams. Soviet Union Pat. No. 530,877 discloses the use of a reducing agent such as reduced iron, stannous chloride, or cuprous chloride to reduce Fe(III) to Fe(II) to facilitate the precipitation of the iron. U.S. Pat. No. 4,533,473 describes the removal of metallic contaminants by contacting the process stream with a dilute aqueous solution of a mineral acid in which the contaminant is generally soluble. The contaminant can then be removed with the dilute mineral acid. Other patents, such as U.S. Pat. No. 4,412,086 and U.S. Pat. No. 4,307,261, describe the use of hydrocarbon oils which are less volatile than the desired products and in which ferric iron is less soluble to separate the ferric iron from the desired products. The more volatile hydrocarbons are removed by flashing or fractional distillation and the iron and less volatile oils are left behind. The iron is not soluble in the remaining hydrocarbon oil and precipitates out and can then be separated.
These techniques are not without problems. The catalyst recovered generally has lost most or all of its catalytic activity. Further, the processes necessary to remove the iron or other catalysts frequently require multiple steps and the building and maintenance of special equipment. Thus, what is needed is a method of removing the ferric iron or other metallic contaminants from chlorinated hydrocarbon process streams that is simple, efficient and economical and that retains the iron in a form with its catalytic activity either unchanged or minimally changed so that the catalyst may be recycled.